1. Field of the Invention
The present invention relates to an antenna apparatus of a portable radio terminal having a radio function.
2. Description of the Related Art
In a portable radio terminal including an antenna and a radio that are integrated as one component, the directivity of the antenna is likely to vary with the shape of a cavity.
It is known that the use of a half-wave (λ/2) antenna reduces current flowing from a feeder line to a cavity. If a half-wave linear antenna is employed, an influence of a cavity is reduced, which is convenient for the design of an antenna system.
Even in the half-wave antenna, however, current is likely to flow into the cavity to make it impossible to form a desired pattern depending on the configuration of a matching circuit.
In most cases, a quarter-wave (λ/4) element is used as the matching circuit for impedance matching between a half-wave antenna and a load. It is known that the quarter-wave-length element has relatively wider band characteristics than the matching circuit of lumped constant elements does. However, there is a problem that current leaks from a feeder point to a cavity when the quarter-wave-length element is inserted between the half-wave antenna and the feeder point. Such a leakage current causes unnecessary electric wave emitted from the cavity.
Conventionally, when the quarter-wave-length element was used as a matching circuit, its wide-band characteristics were prioritized and a leakage of current to the cavity had to be ignored. In other words, an attempt to optimize the quarter-wave-length element was made to obtain wide-band characteristics, not to reduce a leakage of current to the cavity.
In Japanese Patent Application No. 11-051462, the inventors of the present invention proposes an antenna apparatus in which a connecting point between a half-wave antenna and a quarter-wave-length element serving as a matching circuit is located in a specific space of a cavity to reduce a leakage of current to the cavity.
FIG. 8 is a schematic diagram of the antenna apparatus proposed in the above Japanese Patent Application. Referring to FIG. 6, a connecting point 65 is located close to the side of a cavity 61 and somewhat below a feeding point 62 within a range of the side. This configuration produces the advantage of reducing a leakage of current to the cavity.
However, the antenna apparatus shown in FIG. 8, of which the quarter-wave-length element serving as a matching circuit has to be encased in a plastic cavity of a cellular phone, the quarter-wave-length element becomes relatively long depending on the frequency and thus requires a large area when it is inserted in the cavity of a cellular phone that steadily decreases in size. Further, the wide-band characteristics of the quarter-wave-length element are likely to deteriorate.